Conventionally, electrodeposition painting has been widely used in painting automobile parts, electrical products, construction materials and the like as well as automobile bodies. An electrodeposition painting system is configured by an electrodeposition step of electrochemically forming a coating on an object to be painted, a washing step of washing off non-electrodeposited paint or the like, and further a baking step of hardening the coating. Generally, a water washing step is configured by a membrane filtration filtrate multistage recovery water washing step and a final water washing step.
The membrane filtration filtrate multistage recovery water washing step is a process in which paint physically adhered to the painted object is washed off by washing the painted object with filtrate obtained by filtering out paint in an electrodeposition bath by a filtration membrane, and also non-electrodeposited paint is recovered to the electrodeposition bath. Further, the final water washing step is a process in which washing for finishing is performed with pure water or purified water (industrial water), and a trace amount of paint or impurity ions that have not been able to be washed off in the membrane filtration filtrate multistage recovery water washing step is washed off. However, water used in washing is discharged outside from the process, as waste water.
FIG. 7 illustrates an example of a conventional electrodeposition paint recovery system. An electrodeposition bath 101 is illustrated in FIG. 7, and a membrane filtration filtrate multistage recovery water washing step is configured by three stages of a spray-type first water washing bath 102, a dip-type second water washing bath 103 and a spray-type third water washing bath 104. Further, the final water washing step is configured by two stages of a dip-type first water washing bath 105 and a spray-type second water washing bath 106. The spray-type water washing bath is a type of water washing bath in which a painted object is washed with water by spraying water for water washing onto the painted object. Meanwhile, the dip-type water washing bath has a larger amount of retained water for water washing than the spray-type water washing bath, and is a type of water washing bath in which a painted object is washed by completely immersing the painted object in water for water washing.
The object to be painted is mounted on a conveyer (not shown), and electrodeposition painting is performed on the object to be painted by immersing the object to be painted in the electrodeposition bath 101. After then, the painted object is washed with water by being conveyed sequentially to the first water washing bath 102, the second water washing bath 103 and the third water washing bath 104 in the membrane filtration filtrate multistage recovery water washing step, and to the first water washing bath 105 and the second water washing bath 106 in the final water washing step. A first membrane filtration apparatus 107 is illustrated. Electrodeposition solution is sent from the electrodeposition bath 101 to the first membrane filtration apparatus 107 through a line 108, and membrane filtration is performed. Concentrated solution that has not passed through the membrane is returned to the electrodeposition bath 101 through a line 109. Filtrate is sent to the last stage of the membrane filtration filtrate multistage recovery water washing step, which is the third water washing bath 104 in the example illustrated in FIG. 7, through a line 110, and used as water for water washing in the membrane filtration filtrate multistage recovery water washing step. The water for water washing in the membrane filtration filtrate multistage recovery water washing step sequentially overflows, one to another, from the third water washing bath 104 to the second water washing bath 103 and to the first water washing bath 102, and used as water for water washing in each of the water washing baths. After then, the water for water washing further overflows from the first water washing bath 102 to the electrodeposition bath 101, and non-electrodeposited paint is recovered. Pure water or purified water (industrial water) is fed, as water for water washing, to the first water washing bath 105 in the final water washing step through a line 113, and pure water or purified water (industrial water) is fed to the second water washing bath 106 through a line 111, and washing is performed. The pure water fed to the second water washing bath 106 overflows to the first water washing bath 105, and is discharged from a line 112 together with purified water fed to the first water washing bath 105.
However, the recovery water washing method by the conventional electrodeposition paint recovery system, as described above, had a problem of an increase in the amount of paint taken outside from electrodeposition painting facilities, because the concentration of non-electrodeposited paint in each of the water washing baths increased in a case where electrodeposition painting was performed on a large amount of object to be painted, or to prevent such a problem, the method had problems of an increase in the amount of purified water and pure water used in the final water washing step and an increased load of wastewater treatment. These problems are solvable by increasing the number of stages in the membrane filtration filtrate multistage recovery water washing step, but new problems of an increase in the cost of facilities and a space for setting arise.
To solve the aforementioned problems, Patent Literature 1 (Japanese Unexamined Patent Publication No. 7 (1995)-224397) proposed filtering water for water washing recovered in the first stage of the membrane filtration filtrate multistage recovery water washing step by an ultrafiltration membrane, and feeding the obtained filtrate to the last stage of the membrane filtration filtrate multistage recovery water washing step. However, in this method, for example, in a case where the water washing bath in the first stage is a spray-type water washing bath, the amount of filtrate obtained by performing ultrafiltration on the water for water washing recovered in the first stage is small. Therefore, there is a problem that the concentration of non-electrodeposited paint in the last stage is not sufficiently lowered.
Further, Patent Literature 2 (Japanese Unexamined Patent Publication No. 2011-99158) proposed taking out water for water washing recovered from a water washing bath provided between the first stage and the last stage of the membrane filtration filtrate multistage recovery water washing step, and after then, performing filtration by an ultrafiltration membrane, and feeding the obtained filtrate to the last stage of the membrane filtration filtrate multistage recovery water washing step.
FIG. 8 illustrates an electrodeposition paint recovery system disclosed in Patent Literature 2. In FIG. 8, the same numbers are assigned to apparatuses that are identical with those of FIG. 7. The electrodeposition paint recovery system illustrated in FIG. 8 is characterized in that a second membrane filtration apparatus 120 is newly provided for the dip-type second water washing bath 103 in the conventional electrodeposition paint recovery system illustrated in FIG. 7. The other features are the same as the conventional electrodeposition paint recovery system illustrated in FIG. 7. Second water washing bath solution is fed to the second membrane filtration apparatus 120 through a line 122, and concentrated solution that has not passed through the membrane is returned to the electrodeposition bath 101 through a line 121. The filtrate is fed to the third water washing bath 104 (the last stage of the membrane filtration filtrate multistage recovery water washing step) through a line 123, and used, as water for water washing, together with filtrate from the first membrane filtration apparatus 107 fed through the line 110.
Further, Patent Literature 3 (Japanese Unexamined Patent Publication No. 2004-149899) proposed filtering electrodeposition solution extracted from an electrodeposition bath by an ultrafiltration membrane, and filtering the obtained filtrate by a reverse osmosis membrane, and feeding the filtrate obtained by the reverse osmosis membrane to the last stage of the membrane filtration filtrate multistage recovery water washing step.